Web λ.0 - A different view on programming

Tuesday, October 18, 2016

Forth is an unusual programming language. To learn it, "you must unlearn what you have learned", as Master Yoda would say. There are many indications that Forth is a programming language of Jedi: it uses postfix notation for expressions (so "a + b" becomes "a b +", which means "to sum receive, a and b you must add" using Yoda's words), it is extremely minimalistic (in most Forths, the language core is written in assembly, and the rest of the language constructs, including conditional, branching and loop instructions, is written in the Forth itself), and it requires long study to understand (even people using Forth in real life to build commercial software sometimes admit, that they are yet to understand it). Also, although Forth has an official ANSI standard, it is so flexible, that most Forth masters tend to build their own light sabres (I mean Forth systems) using the base Forth only as a foundation. Such thing as MANX Musical Forth (an extension of the regular Forth designed specifically to work with MIDI) or J1 Forth (an FPGA implementation of a stack-based CPU) are nothing unusual in the world of Forth. Forth is also complete: it's an operating system, a runtime environment, and an interactive compiler - all that in just a few kilobytes (not megabytes!) of code.

Forth had its best days during the early computers era, since it is extremely well suited for machines with very limited resources. Some of the 8-bit home computers, like British Jupiter ACE or French Hector HRX, used Forth as their operating system. Recently, it has been used successfully in XO-1 laptop's firmware.

Saying all that, no wonder that there is a Forth for Arduino. No wonder that there is more than one. No wonder that most of Forths for Arduino get rid of the bootloader and take full control of the hardware. I decided to try some of them and describe my very short and very subjective experience here.

First, you need an Arduino Uno, or Arduino Nano with Atmega328 CPU, a programmer (I recommend USBasp since it's inexpensive and easy to use) and software that allows programming Arduino board directly. If you have Ubuntu Linux, you can install it with the following command:

Connect USBasp to the ISP pins of Arduino and put it into the USB port in your computer. You can now upload software straight to the Atmel chip using avrdude command line tool. It is important that you understand what this tool does, before you start fiddling with command line switches, because you can brick the board if you misuse them (this applies primarily to hfuse). I recommend reading Martin Currey's Arduino / ATmega 328P fuse settings if you want to use settings other than provided in this article.

AmForth 6.3

Let's start with AmForth. Download the AmForth distribution archive, and extract appl/arduino/uno.hex and appl/arduino/uno.eep.hex files. They contain compiled binaries in the form of human readable Intel HEX format. Connect the programmer and run the following command (the whole command should be a single line):

After a while, AmForth will be uploaded to Arduino, and the board will reset. Now, connect the Arduino with your computer using mini USB cable and open a terminal program with the following parameters: baud rate 38400, 8 bits, no parity, and 1 stop bit. For Windows, you can use Putty, just select connection type "serial" and a suitable COM port. In Mac OS X and Linux you can use screen, but the connection port depends on the chip your board uses for serial communication. If you have original Arduino, or a more expensive clone, which uses FTDI chip, the command will look similar to:

screen /dev/ACM0 38400

for Linux, and

screen /dev/tty.usbmodem1421 38400

for Mac OS X. In your case device port can be different, depending on which port Arduino is connected to, but it should follow the general pattern of /dev/ACM... for Linux and /dev/ttyusbmodem... for Mac.
With cheaper Arduino clones, using CH340 chip for serial communication, the command will look like:

screen /dev/ttyUSB0 38400

for Linux, and something similar to:

screen /dev/cu.wch\ ch341\ USB\=\>RS232\ 1420 38400

on Mac OS X. To disconnect from screen, use the following key combination: ctrl + a, ctrl + backslash, enter.

If everything goes well, you can start using Forth on your Arduino. For example, you can enter your first program, which calculates the greatest common divisor of two numbers:

: gcd ( a b -- gcd )
begin
dup
while
swap over mod
repeat
drop ;

Forth instructions are called words and are stored in a dictionary. The first line defines a word gcd (colon is the beginning of a word definition), and contains a comment (in brackets) which says that the word expects two values as input (a and b) and produces one value as output (gcd). The names used in comment can be anything, since all values in Forth are put on, and taken from, an anonymous data stack.
Words begin and repeat denote a loop. Within the loop, the current value located on the top of the stack is duplicated. The word "while" takes a value from the top of the stack and checks whether it is false (zero) or true (any value other than zero). If it's true, the loop continues. Because "while" consumes the value it takes from the stack, we need word "dup" before it. Otherwise, the word "while" would eat up all the values from the stack.
Next we swap the top two values on the stack and replicate the lower one to the top. Basically, we take "a b" series and create "b a b" from it. The word "mod" takes two values from the stack, divides them, and puts back the remainder. Now the stack looks like this: "b remainder". The loop is repeated, so the remainder is duplicated and evaluated. If it's not zero, the loop continues, otherwise it is removed from the stack (by the word "drop") and the value remaining on the stack is the final result. Semicolon ends the word definition.

To test how it works, input the following command:

15 25 gcd .

It puts 15 and 25 on the stack, executes "gcd" word and prints (dot means "print on the screen") the top value from the stack, which happens to be our result.

I wanted to know how fast AmForth is, so I wrote a simple benchmark, which calculates the greatest common divisor for all combinations of numbers from 0 to n:

: bench ( n -- )
dup 0 do
dup 0 do
j i gcd
drop
loop
loop
drop ;

In Forth "do/loop" is a loop which needs two values on the stack - the starting value, and the ending value. With "10 0 do ... loop" you repeat the loop from 10 down to 0. To execute the loop n times (as you see in the comment, "n" is expected to be on the stack when you run "bench") you need to duplicate it with "dup", then put 0 on the stack, and then call "do" which will consume those values. But because "n" was duplicated, it is still on the stack, and can be used in the inner loop. Finally, we calculate the greatest common divisor on the current counters of the inner and the outer loop ("i" and "j"), but since we don't need the result and don't want it to remain on the stack and affect the loops, we need to "drop" it.

The following test

200 bench

takes about 8 seconds on AmForth to complete. It is a very good result comparing to other Forths I tested.

Flash Forth 5

Installing Flash Forth also requires a programmer. You also need avr/hex/ff_uno.hex file, which you can upload to the board using USBasp with:

You can communicate with Flash Forth the same way as with AmForth, but using different baud rate:

screen /dev/ACM0 9600

Flash Forth comes with separate math library, so to be able to define "gcd" word in Flash Forth, you need to download it from http://flashforth.com/math.txt and rewrite or upload it via the terminal. You need to be careful with uploading, though. If you just copy and paste the whole file in the terminal you will overrun the input buffer and Flash Forth will start returning errors. The same applies to uploading code to other Forths, too. It's best to copy the code definition by definition or to use special software, which slows down the transmission (for example, iTerm has a special paste option "Paste Slowly").

Flash Forth does not support "do/loop", and uses "for/next" instead. So the word "bench" has a slightly different definition:

Word "for" takes only one value from the stack, and always counts down to zero. Also, because there is no "do/loop" in Flash Forth, there is no "i" and "j" either, and you need to copy the current loop counter from the return stack (which keeps track of the program execution) to the data stack with "r@". Except from the syntax, the loop construct remains the same as with AmForth.
However, Flash Forth turns out to be much faster. Running "200 bench" takes about 4 seconds to complete, which is twice as fast as with AmForth.

328eForth 2.20

It's another Forth for Arduino, which is a direct descendant of renowned eForth. Its main advantage is simplicity - the whole source code fits in one file of Atmel assembly, and the compiled hex is only 14 kilobytes long. Unfortunately, the original project page is no longer avaiable, but you can download the source code of version 2.20 from this Github repository.

The repository does not provide a compiled binary, though, so you need to make it yourself. Fortunately, it's quite easy - all you need is Atmel Assembler, which you can find on Sourceforge. It's a Windows executable, but it works pretty well with Wine, so you can compile 328eForth on Linux and Mac with no problem. Put my_forth.asm file in the avr8/Atmel directory and run the following command:

avrasm2.exe -fI -I Appnotes2/ my_forth.asm

You should now have the my_forth.hex file, which you can upload to Arduino with:

I love this Forth implementation for its simplicity, but unfortunately it is quite slow and buggy. The "gcd" word does not work properly, because the word "mod" is broken and returns wrong results. Also, the benchmark executes in 27 seconds with 328eForth, comparing to 8 with AmForth and 4 with Flash Forth. To make things worse, the project page and documentation are missing and can be reached only partially, through the Wayback Machine.

Yaffa Forth 0.6.1

This Forth is different. Yaffa Forth is written in C and can be uploaded to the Arduino board like any regular sketch, with Arduino IDE. It's a good option for people who don't have a programmer or only want to give Forth a short try. It also provides standard Arduino interface with words such as "pinMode", "digitalRead/digitalWrite", "analogRead/analogWrite", etc. AmForth, Flash Forth and 328eForth don't use Arduino libraries, so you have to talk to the pins directly via I/O ports. Also, because Yaffa's source code is very clean and well documented, you can easily extended it with new words which can use existing Arduino libraries written in C.

Yaffa Forth has some deficiencies, though. Because it works on top of virtual machine written in C, which itself also needs memory, it has less space available for the stack, which can be especially painful on Arduino Uno or Nano (they both only have 2kB RAM). Also, it is much slower than previous Forths - the benchmark code identical to AmForth's takes 70 seconds to run, which makes it about ten times slower than AmForth and almost twenty times slower than Flash Forth.

There is also one more important difference between aforemetioned Forths and Yaffa Forth. AmForth, Flash Forth and 328eForth store all user-defined words in flash memory, together with the main dictionary (in 328eForth a new word is defined in RAM, but must be copied to flash with word "flush" before it can be used). This means that if you turn off the power, your definitions remain in Arduino's memory. With Yaffa Forth, all new words are stored in RAM and disappear once you turn the board off or press the reset button. If you want to store your definitions for future use, you must write all of them in EEPROM with eeLoad -> code -> ctrl + z). It's because Yaffa Forth relies on Arduino bootloader, which prevents user applications from writing directly to flash. On the other hand, EEPROM memory can handle almost ten times as many write cycles as flash before it wears off, so in this respect it may be more hardware friendly to use Yaffa Forth than its counterparts.

Tuesday, September 20, 2016

Forth is a very powerful programming language. It can scale from very low level, near-assembly code, which you can run on Arduino Uno with 2 kilobytes of RAM, to very high level, domain-specific language, suitable for modern home computers (you can even implement object-oriented Forth in the Forth itself).

Due to its inherently low-level nature, Forth is not very popular among contemporary programmers. It is often considered a high level assembly - it stores and manipulates data using anonymous data stack (and optional floating point stack), and variables are nothing more than labelled memory cells. But at the same time Forth provides a set of tools to create building blocks, which can be used to make even the most sophisticated software (like satellite antenna controllers). Developers who understand how a CPU works and are not afraid to juggle with bytes, usually have no fear of Forth and find it a very smart and productive tool.

There are many implementations of Forth for almost all existing operating systems (many of them are self-contained and can run without an operating system at all, like amForth or Jupiter ACE's Forth). Among them there is excellent, and still actively maintained, DX-Forth for CP/M. Thanks to amazing work of Marcelo Dantas and his fantastic CP/M emulator for Arduino Due, and my humble contribution in the form of CP/M Arduino Interface, you can now enjoy writing software for Arduino using DX-Forth.

RunCPM emulator contains an example Forth application, PHOTOLED.4TH. It begins with a couple of Forth words, which define the Arduino interface. The first one is pinmode:

: pinmode ( val pin -- ) 8 lshift + 220 bdos drop ;

From its declaration, pinmode expects two bytes on a stack, the first one being a value to assign to the pin, and the pin number. It uses a word bdos, which DX-Forth defines as follows:

BDOS ( DE u -- A )
Perform CP/M BDOS call number u. DE is the value passed to the DE register. Return the contents of the A register.

So first we need to put on the stack some value, which will then be passed to CPU register DE, and then a BDOS function number. Question is, what should be passed as DE and the call number? The answer can be found in the Arduino interface description:

By convention, you pass BDOS function number in CPU register C, and arguments in registers D and E. All those registers are 8-bit, but can be combined in 16-bit pairs: BC, DE and HL. So for the Forth word bdos we need two numbers to be put on the stack:
- First, a 16-bit number with pin number in high 8-bits and mode (0, 1 or 2) in low 8-bits. This number will be passed to CP/M through CPU register pair DE.
- Second, number 220 as the function call.
For example, to set pin 13 into output mode, we need the following command in Forth:

3329 220 bdos

3329 is a product of 13 * 256 + 1. Multiplication by 256 is the same as shifting 8 bits left. The pinmode does exactly that: it takes the pin number from the stack, shifts it 8 bits left, add the next value from the stack (val) and calls BDOS function 220:

: pinmode ( val pin -- ) 8 lshift + 220 bdos drop ;

The result of the call can be ignored, so the return value is simply dropped.

With din we want to preserve the result of bdos call on the stack, so we don't drop it:

: din ( pin -- n ) 8 lshift 221 bdos ;

Additionally, with word ain, we use fdos instead of bdos:

: ain ( pin -- n ) 8 lshift 223 fdos drop ;

It's because function number 223 makes a call to AnalogRead, which returns a 10-bit value and needs more than one 8-bit register to pass the result back to Forth. The value of AnalogRead is returned by the Arduino interface in register pair HL, and fdos returns this pair along with register A, unlike bdos, which returns only A:

FDOS ( DE u -- HL A )
Perform CP/M BDOS call number u. DE is the value passed to the DE register. Return the contents of the HL and A registers.

Since we need HL only, we can drop what has been returned in A, and leave only HL on the stack.

Now we can define words which use pinmode, din, dout, ain and aout. For example to turn a led on and off we can define the following words:

Both words require the pin number to be on the stack. They both put the pin to output using pinmode and then set its state to HIGH (1) or LOW (0) respectively.

A more complicated example requires a simple circuit to be built. It should consist of a LED and a LDR (photoresistor) according to the following diagram:

The LED is connected to pin D9 (it can't be any digital pin, it must be PWM capable) through a 68 ohm resistor (unlike Uno, Arduino Due outputs 3.3V), and the LDR is connected to pin A8 and uses a 10k ohm pull-down resistor. We want a LED to light up when it gets darker, and get dim when it's not. Because, as you remember, analog input can return values with 10-bit resolution, and digital output accepts only 8-bit values, we need to calculate the LED intensity according to the following formula:

LED = 255 - (LDR / 4)

This can be achieved through a new word fade (it uses shifting 2 bits to the right to do a division by 4):

: fade ( led ldr -- ) ain 2 rshift 255 swap - swap aout ;

Now when you run the following command in DX-Forth:

9 8 fade

the LED intensity gets adjusted according to the current value returned by the LDR.

Of course it is tedious to run it by hand. We can define a new word, which will run fade in a loop:

: run begin 9 8 fade 100 ms key? until ;

It runs 9 8 fade with 100 millisecond delay until any key gets pressed. The final result looks like this:

If the LED responds too slow, or flickers, you should try tuning the delay. You can also get rid of it completely:

Wednesday, September 14, 2016

RunCPM, a CP/M emulator for Arduino Due, can now use Arduino's analog and digital pins thanks to five additional BDOS calls, which virtually makes it the first (and so far the only one) fully fledged, professional operating system for Arduino. You can write brand new, or modify any existing, CP/M software, using any programming language available for CP/M platform, to interface Arduino devices. The only requirement is that it should allow you to call arbitrarily chosen BDOS functions (natively or via inline assembly code), because RunCPM uses newly defined routines (220 - 224), unused by standard CP/M implementations.

RunCPM includes a few sample programs in Assembly, Microsoft Basic, Hi-Tech C, Turbo Pascal and DX-Forth to get you started. I also prepared a short video tutorial showing the Turbo Pascal example in action:

A few tips:

Because analogRead returns values that won't fit in a single byte, when calling function 223 in Turbo Pascal you should use BdosHL instead of Bdos to get the value returned in HL register pair, like this:

Saturday, July 16, 2016

CP/M, created by Gary Kildall, was for early computers what Android is for today's smartphones and tablets. It was an operating system that could be easily customized to run on any machine with Intel 8080 or Zilog Z80 processor, and was de-facto standard during 1970s and 1980s, running on hundreds of computers coming from different manufacturers, from Altair 8800 to early IBM PCs. Unfortunately, computers that can still run CP/M are very rare and expensive, and their modern counterparts, made by hobbyists, are either also quite expensive (like Altair 8800 clone) or require some more skills and effort than just simple soldering to build (like Grant Searle's Multikomp).

Luckily for all who want to have their own CP/M computer, there is an Arduino based solution, which is quite cheap and very simple to build. All you need is Arduino Due or its clone and an SD shield (like this one), which you have to fit on top of your Arduino board.

Now you need to download Marcelo Dantas' RunCPM, which is a multiplatform CP/M emulator, and upload it to the board with Arduino IDE. I managed to build RunCPM without problems on Ubuntu 12.04 using version 1.6.7 for Linux 64bits, just remember to install support for Arduino Due with Boards Manager under Tools menu. Next, connect the board to your computer with a micro USB cable inserted into the programming port of Arduino, and then select "Arduino Due (Programming Port)" for the Board and the Port (remember: Arduino must be connected to the computer). Open RunCPM.ino project, select "Verify/Compile" under the Sketch menu to produce the binary image, and then select "Upload" to flash the board with it.

Now, when you have a fully fledged Z80 board, all you need is the CP/M operating system. RunCPM comes with the most popular version 2.2 of CP/M. To run it, format an SD card as FAT32, and put the CPM22.BIN from RunCPM repository in the main folder of the card. Also create folders "A" and "B" - these will be your disk drives, everything you put in there will appear to the system as located on disks "A:" and "B:" respectively. You can create folders from A up to P (as this is the limit of CP/M itself). Now turn off the Arduino, put the card into the SD card shield, and connect it back. You now have the smallest CP/M computer in the world up and running.

Ok, the system has started, but how do you talk to it? Well, you need a terminal, just like for a real CP/M machine! Depending on your operating system, you will need a terminal software. For Windows I recommend PuTTY, in Linux you can install minicom package. Set serial transmission parameters to 8N1, speed to 9600 bauds, and select a serial port to which your CP/M machine is connected. In Ubuntu it's:

minicom -b 9600 -o -D /dev/ttyACM0

With MacOSX it's even simpler, just start typing "screen /dev/tty.usb" from the terminal and press TAB to unfold the device full name. In my case the full command looks as follows:

screen /dev/tty.usbmodem1421

If you configured the connection correctly, you will now be able to work on your Arduino board like on a real CP/M computer.

Just remember to disconnect when leaving the terminal session (ctrl+a and then z in minicom or ctrl+a and then ctrl+\ in OSX) otherwise you may get some strange errors when trying to reconnect.

RunCPM allows you to have your own, modern, inexpensive CP/M computer, running at the speed of original Z80 based hardware, and use all the huge library of CP/M software with it. At the same time, you can connect to it with USB instead of a serial cable, you can easily exchange software with an SD card, it takes very little power, and last but not least - it easily fits in a pocket.

Thursday, April 21, 2016

NetUSBee is a network card for 16-bit Atari computers, which is compatible with modern ethernet devices. To make it work under plain TOS you need Sting drivers; you can find those drivers, as well as the installation manual, here. Unfortunately, the manual covers only peer to peer connection, and it's not enough when you want to connect your computer to the Internet. Two importand pieces missing are gateway and DNS configuration.

Gateway, as the name suggests, is a network device which passes traffic from your host to the Internet, and vice versa. If you configure an Atari and a PC as specified in the manual, the Atari will have an IP address 192.168.255.1, and the PC's IP address will be 192.168.255.2. If you enable Internet connection sharing in the PC, it will become a gateway for the Atari, and the Sting drivers must be properly configured for it.
To do this, run a text editor (I strongly recommend Everest for this purpose), open file C:\STING\ROUTE.TAB and scroll to the end. You will see the following lines:

This is so called routing table; it tells the host which gateway it should use when communicating with a specific network. Because the entries above make no sense for NetUSBee configuration described in the manual, you should remove them. Instead, replace them with the following entry:

0.0.0.0 0.0.0.0 EtherNet 192.168.255.2

This defines so called "default gateway". What it says is that for any IP address ("0.0.0.0" means "any") in any subnetwork (again: "0.0.0.0") the traffic should be passed through the computer connected to the ethernet interface, with IP address "192.168.255.2" (it's the PC's address).
Now very important note: you should use tabs to separate the column entries, so the line would be typed in like this:

0.0.0.0<TAB>0.0.0.0<TAB>EtherNet<TAB>192.168.255.2

If you use spaces, it will not work, and some editors silently replace tabs with spaces - this is why I recommend Everest (but QED will work as well).

Second thing you need to set up is a Domain Name Server configuration. DNS is a service used to translate human readable names into IP addresses - when you type in "blogspot.com" in your browser, it will contact a DNS server first, to ask what IP number hides behind it.
To configure DNS in Sting, open C:\STING\DEFAULT.CFG in text editor, find the line which says:

NAMESERVER =

and enter your Internet provider's DNS addresses. If you don't know what they are, you can use Google's:

NAMESERVER = 8.8.8.8, 8.8.4.4

Save modified DEFAULT.TAB and ROUTER.TAB files and restart the computer. If you configured Internet Connection Sharing properly in the PC, you will be able to use various Internet applications on your Atari.

Monday, February 29, 2016

There are more and more examples of privacy violation in the Internet. Even in supposedly democratic countries, the governments want to know what the citizens are up to, "just in case". For example, in my country the police have recently been given the power to eavesdrop phone calls and intercept any forms of electronic transmission, without prior permission from the court.
Thinking that you have nothing to worry about if you do nothing wrong is naive. Any information gathered about you can be sooner or later used against you. It can also leak, be stolen, or even sold by a corrupted police officer. Remember, that your home network consists of many devices, not only your personal computer (or computers). Your phones, tablets, TV boxes, and sometimes even your refrigerator as well as other home appliances, constantly transmit various data over the network. Many of them use RESTful APIs over unsecured HTTP channels, because it's simpler and helps keeping the costs low. But this is only part of the problem. Even if you make sure you use only encrypted HTTPS protocol, your software still needs to perform unencrypted DNS queries to translate domain names to IP addresses. This way anyone eavesdropping your network traffic can gather information about the sites you visted, how often and for how long. This so called "metadata" can reveal much more information about you than the actual content you view with your Internet browser. Metadata can tell a lot about your interests, habits, in which banks you keep your money (because you visit their sites often), what you buy and where you do your shopping, even where you plan to spend next weekend (because you just checked weather forecast for this place with your mobile phone). It can even be used to check if you are at home and whether it can be safely robbed. You can be profiled quickly and without much effort, just by intercepting transmission from you home router. Moreover, your traffic can be redirected (through DNS spoofing) to false web servers, to make you reveal some sensitive data, like logins and passwords. Also, your devices automatically synchronizing time over the Internet can be tricked into using a particular timestamp. The timestamp is a crucial element of digital cryptography: controlling the clock of your computer will make it generate predictable cryptographic keys, which then can be easily broken.

So, how can you protect yourself? My solution was to built a personal Virtual Private Network. I have a VPN server hosted in the cloud, and a home router configured as VPN client, passing all my Internet traffic through that server over an encrypted "tunnel". Of course it makes my Internet connection a little slower, because every data packet, including metadata queries (like DNS and NTP), must be encrypted and transferred to the cloud, where it "jumps out" and begins its regular way through the network, but this way my network traffic cannot be eavesdropped. The server is hosted in another country, so the government can no longer watch me "just in case", because they need a court permission to claim my data from a foreign cloud provider.

How is this better than TOR, or any other peer-to-peer network? - you may ask. With TOR, the idea is that your data travels around the world through other people's computers, but also other people's data travels through yours, and you have no control over it. Frankly speaking, I don't feel comfortable knowing that my computer can be indirectly used by criminal offenders for their illegal activities.

I used tinc to build the VPN. It's fairly easy to configure and available for many operating systems. I used Ubuntu for the server, and OpenWRT router for the client, but you can use a different configuration (for example an old PC with Linux as a router).

1. Server configuration.

First, you need to set up a server. Tinc has very minimal hardware requirements (a few megabytes of disk space and memory), and I use it successfully with Ubuntu 14.04 installed on a virtual server with 512MB RAM. First, you need to be root to perform all necessary tasks. Swith to user root with:

sudo bash

Then install the tinc package:

apt-get install tinc

Next, create a proper directory tree and configuration files using the following commands:

This makes the basic scaffolding of your VPN server configuration. Now you must add the server's public IP address to the /etc/tinc/vpn/hosts/server configuration file (replace "xxx.xxx.xxx.xxx" with the public IP address of the server):

echo 'Address=xxx.xxx.xxx.xxx' >> /etc/tinc/vpn/hosts/server

Notice ">>" instead of ">" when using this command!
The file /etc/tinc/vpn/hosts/server should now have the following content:

Subnet=10.8.0.0/24
Mode=switch
Address=xxx.xxx.xxx.xxx

Where, again, "Address=" field contains the public IP address of your server.
Now you need to generate an encryption key used to protected data transmission over the VPN tunnel:

tincd -n vpn -K

Confirm the defaults pressing Enter. After this operation you will see a new file /etc/tinc/vpn/rsa_key.priv containing the private key, and a new section in /etc/tinc/vpn/hosts/server file containing the public key. As their names state, the public key can be visible to anyone, and the private key must remain secret and cannot be revealed.

You can now start tinc server (as user root) with:

tinc -n vpn

Now you should be able to list the new network interface with:

ifconfig vpn

And it sould have a section saying "inet addr:10.8.0.1". You can turn it off for now with:

tincd -n vpn -k

Notice lower case "-k", not uppercase "-K", which was used to generate the key.

2. Client configuration.

Now for the router (client) configuration, you also need to install tinc package. With OpenWRT you can do it either through the web interface or via the command line with the following command:

opkg update && opkg install tinc

With Ubuntu as the client the procedure is the same as with the server:

sudo apt-get install tinc

Now you need to configure the client. On the router (or the computer acting as a router) switch to user root with:

You are almost done. Your router is now the client of the VPN network, and you need to make sure that the server and the client know about each other, so they can communicate. To achieve this, you need to copy /etc/tinc/vpn/hosts/server file to the client, and /etc/tinc/vpn/hosts/client to the server, so that the contents of the /etc/tinc/vpn/hosts directory is the same on both hosts and contains both client and server files.
Now start tinc both on the client and the server (again, as user root) with:

tincd -n vpn

After a short while the connection should be established and you should be able to ping the server (10.8.0.1) from the client (10.8.0.2) and vice versa. If something goes wrong, you can start tinc with debugger to see what's going on:

tincd -n vpn -d -D

The messages are elaborate enough to allow you quickly locate the source of the problem.

3. Routing setup.

You are almost done here. The client and the server can see each other, but you want to pass all your traffic through this connection, right? So there are two more things to be done.

First, you need to configure the server to forward the traffic between the VPN and the outside world:

The above commands should be run every time you start the server, so you can add them to /etc.rc.local startup script (along with the command to start tinc).

Second, on the client side, you need to modify the routing table to make the VPN server a default gateway for all the internet traffic passing through the client (i.e. your router), except the VPN server itself. So, check the routing table on your VPN client with:

Look at the Gateway column in the line where the Destination says 0.0.0.0 (meaning: any address) - it's the default gateway. In this case it's 5.10.64.1 and it's the address of you nearest outside world neighbour, probably your Internet provider's modem or router. You need to make sure that you leave this route open for the connection between your client and your server to work.
Do the following magic commands (replacing 5.10.64.1 with your own default gateway, and xxx.xxx.xxx.xxx with the public IP of your VPN server):

What happens here is that you leave the traffic to your VPN server via the old route, and replace the gateway for all other traffic with the encrypted VPN. If you did it properly, you should now be able to ping any Internet address from your client. Also, the traceroute (or any other similar utility) should show your traffic going first through the tunnel:

If you don't want to modify the routing table by hand every time you start the router, you can add the commands to /etc/tinc/vpn/tinc-up script. Don't add them to /etc/rc.local, because that script is executed early on startup, when 10.8.0.1 address may not yet be available, and the commands will fail.

Now the last step. You need to allow traffic between your local network and the VPN, similar to what you did on the server side. Assuming your router's (the VPN client's) LAN address is 192.168.1.1 (it's a common default for most routers), you need to run the following commands:

Now connect to your router and go to any address from https://www.google.pl/search?q=what+is+my+ip. You should see the public IP of your VPN server, not your home router. It indicates that all your Internet traffic now goes through the VPN.

And finally, one more thing. If you haven't done it yet, change the primary DNS server on your router to 8.8.8.8 and secondary to 8.8.4.4 (they are Google DNS servers). Otherwise all your DNS queries will go through the VPN server back to your Internet provider, and you don't want that to happen, do you?

Saturday, January 23, 2016

You cannot boot current versions of Raspberry Pi from mediums other than an SD card, which means that for some space consuming operating systems, like Raspbian, you need at least a 4GB or 8GB card. But it doesn't mean that if you have a 1GB or 2GB SD you cannot use a larger system on Pi. You just need a memory stick with sufficiently high capacity.

The trick is simple. Write the operating system image on a pendrive instead of an SD card. Format the card just like you do for everyday use with a camera or smartphone, i.e. just create a single partition with regular FAT file system on it. Now mount the memory stick and you will notice that the first partition is also a FAT parition with a couple of different files - they are required to boot the operating system. Copy all files and folders from the stick to the card. Now edit cmdline.txt file located on the SD card (this is important, remove the pendrive before editing to make sure you modify the right file). The file contents will be similar to this:

It's the arguments passed to the kernel on boot up. The root parameter tells the kernel where the main file system is located. Change it from root=/dev/mmcblk0p2 to root=/dev/sda2 and save the file on card. Connect the card and the pendrive to the Pi and turn on the power. It will load the kernel from the card, but will mount as root folder the second partition of the memory stick, containing the actual operating system (and recognized by Linux on boot up as /dev/sda2).

I used this trick to run Minibian, having a few memory sticks and just one small 128MB SD card at my disposal. If the Pi doesn't boot from /dev/sda2 make sure you don't have any other usb storages connected, otherwise try a different device instead of /dev/sda2, like /dev/sdb2, /dev/sdc2, etc.